Why Are Your Business Emails Going to Spam? 10 Email Deliverability Rules for 2026

Your sending platform may report that an email was successfully delivered, while the customer says it never appeared in their inbox.

Both statements can be correct.

A successful SMTP delivery means that the receiving server accepted the message. It does not mean that Gmail, Outlook or Yahoo placed it in the primary inbox. The message may have reached spam, been delayed, been rate-limited or been rejected after authentication checks.

In 2026, inbox placement depends on more than a well-written subject line. Mailbox providers evaluate sender authentication, domain and IP reputation, complaint rates, list quality, sending
consistency, unsubscribe processes and the way transactional and marketing traffic are separated.

Email delivery vs. email deliverability

Email delivery refers to whether the recipient’s mail server accepted the message.

Email deliverability refers to where that accepted message was placed and whether the recipient could realistically see it.

A campaign can therefore show a high delivery rate while producing poor results because a significant share of accepted messages was filtered into spam.

Infobip describes deliverability as the result of several signals, including SPF, DKIM and DMARC authentication, sender reputation, bounce and complaint rates, list quality, engagement and consistent sending behaviour.

What changed for email senders?

Gmail and Yahoo began enforcing stronger sender requirements in 2024. Microsoft later introduced stricter authentication requirements for high-volume domains sending to Outlook.com, Hotmail, Live and MSN addresses.

By 2026, these controls are no longer optional enhancements for serious business senders. They are part of the basic operating model for transactional and marketing email.

Gmail requirements

Gmail requires all senders to use SPF or DKIM at a minimum, along with valid DNS, encrypted transmission and standards-compliant message formatting.

Senders delivering more than 5,000 messages per day to personal Gmail accounts need SPF, DKIM and DMARC, alignment between the visible From domain and SPF or DKIM, and one-click unsubscribe for marketing and subscribed messages.

Google recommends keeping user-reported spam rates below 0.1% and preventing them from ever reaching 0.3% or higher.

Outlook requirements

Microsoft treats a domain as a high-volume sender when it sends at least 5,000 messages per day to Microsoft consumer email services using the same domain in the From address.

SPF and DKIM must both pass. The domain must also publish DMARC, and at least SPF or DKIM must align with the From domain.

Microsoft’s current support guidance states that non-compliant messages may be rejected with the 550 5.7.515 error.

Yahoo requirements

Yahoo requires all senders to authenticate through SPF or DKIM, maintain valid forward and reverse DNS and keep complaint rates below 0.3%.

Bulk senders need SPF, DKIM and DMARC with domain alignment. Marketing and subscribed messages must support easy unsubscribe, while unsubscribe requests should be processed within two days.

Yahoo also recommends separating bulk marketing traffic from transactional messages, alerts and individual user mail.

Rule 1: Send from a domain your business controls

Do not run critical transactional or marketing email from a free personal address or a domain whose DNS records you cannot manage.

Use a company-controlled sender such as:

[email protected]

Owning the sending domain allows your business to configure authentication, review DMARC reports, manage reputation and protect the brand against spoofing.

The display name and From address should clearly identify the business. A recipient should immediately understand who sent the message and why it was received.

Rule 2: Configure SPF correctly

SPF publishes the servers and IP addresses that are authorized to send mail on behalf of a domain.

When a provider receives a message, it can check the domain’s SPF record to confirm that the sending source is approved.

Common mistakes include:

• Publishing multiple SPF records for one domain.
• Forgetting to add a new email platform.
• Leaving a discontinued provider in the record.
• Exceeding the SPF DNS lookup limit.
• Authorizing more infrastructure than the business needs.

Your record must cover every legitimate sender, including the Email API, CRM, billing platform, customer-support system and internal applications.

Rule 3: Sign every message with DKIM

DKIM adds a cryptographic signature to the message. The receiving provider uses the public key published in DNS to verify the sender and confirm that signed content was not altered in transit.

Businesses should:

• Create a DKIM key for each sending platform.
• Publish the public key in DNS.
• Enable signing within the sending service.
• Use a suitable key length.
• Rotate keys under an established security policy.
• Avoid relying on one old key across every system.

Yahoo requires a DKIM key length of at least 1024 bits and recommends 2048 bits where possible.

Rule 4: Publish DMARC and achieve alignment

DMARC connects the visible From domain to the SPF and DKIM authentication results.

It is not enough for SPF to pass for an unrelated domain that the recipient never sees. At least the SPF or DKIM organizational domain must align with the From domain.

A business can begin with a monitoring policy:

v=DMARC1; p=none; rua=mailto:[email protected]

This enables reporting so the company can identify every platform sending on its behalf.

After correcting legitimate sources, the policy can move gradually to:

p=quarantine

and eventually:

p=reject

Do not move to rejection before checking billing, marketing, support, HR and internal application senders.

Microsoft requires high-volume senders to publish DMARC with at least p=none, while SPF and DKIM must pass and alignment must be achieved.

Rule 5: Separate transactional and marketing email

Do not send every email type through the same domain, DKIM identity and sending stream.

A company might use:

• transactional.company.sa for receipts, orders and password resets.
• marketing.company.sa for campaigns and newsletters.
• support.company.sa for customer-service tickets.
• security.company.sa for account and security alerts.

Separation reduces the risk that a poorly performing campaign will damage the delivery of invoices, password resets and important account notifications.

Yahoo recommends keeping bulk and marketing traffic separate from transactional mail, alerts and individual user messages.

A dedicated IP is not automatically required for every small sender. Separation can also be achieved with dedicated subdomains, DKIM domains and sending streams.

Rule 6: Implement one-click unsubscribe

A visible unsubscribe link in the message body does not always satisfy the technical one-click unsubscribe requirement.

Marketing messages should include headers such as:

List-Unsubscribe: <https://company.sa/unsubscribe/token>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

RFC 8058 defines how these headers allow a mailbox provider to perform an unsubscribe request through an HTTPS POST. The relevant headers must be covered by a valid DKIM signature.

A clear unsubscribe link should also appear in the email body, and requests should be processed promptly.

The one-click requirement is primarily intended for marketing and subscribed messages. Yahoo confirms that it does not apply to transactional messages such as order confirmations and password resets.

Rule 7: Send only to people who agreed to receive email

Perfect DNS authentication cannot protect a list whose recipients do not want the messages.

Avoid:

• Purchased lists.
• Scraped addresses.
• Partner lists without an appropriate basis.
• Preselected subscription boxes.
• Adding every customer to marketing campaigns automatically.
• Increasing frequency without informing subscribers.

Confirmed or double opt-in can improve list quality by requiring the recipient to verify the subscription.

Saudi Arabia’s PDPL Implementing Regulation covers direct marketing through electronic communication and requires controllers to stop related marketing when the data subject withdraws consent. The PDPL also regulates the use of personal contact details, including email, for advertising and awareness materials.

Specific legal requirements should be reviewed with the organization’s legal or data-protection team.

Rule 8: Clean lists and process bounces

A hard bounce generally means that the address is permanently invalid.

A soft bounce indicates a temporary issue, such as a full mailbox, unavailable server or temporary receiving restriction.

Businesses should:

• Suppress hard-bounced addresses.
• Monitor repeated soft bounces.
• Stop sending to spam complainants.
• Synchronize unsubscribe requests across systems.
• Review long-term inactive subscribers.
• Protect signup forms against automated abuse.
• Validate address formatting during registration.

Yahoo recommends monitoring hard and soft bounces, removing invalid recipients promptly and using confirmed opt-in to improve list quality.

Rule 9: Warm up new domains and IP addresses gradually

A new domain or IP address has no established reputation. Sending hundreds of thousands of messages immediately gives mailbox providers little evidence that the traffic is legitimate.

Start with the most active and engaged recipients, then raise volume gradually while monitoring:

• Bounces.
• Complaints.
• Temporary deferrals.
• Engagement.
• Gmail, Outlook and Yahoo errors.
• Domain and IP reputation.

Avoid sudden changes, such as moving from several thousand messages per month to hundreds of thousands in one day.

Yahoo warns that a sharp increase in sending activity can resemble a compromised sender and recommends planning campaigns and controlling outbound volume.

Rule 10: Monitor reputation and complaints continuously

Do not wait for customers to report missing email.

Track the following for every domain and sending stream:

• Delivery rate.
• Hard-bounce rate.
• Soft-bounce rate.
• Spam-complaint rate.
• Deferral rate.
• Unsubscribe rate.
• Delivery latency.
• Errors by mailbox provider.
• Successful completion by message type.
• Daily sending volume.
• Sudden engagement changes.

For Gmail, aim to remain below 0.1% user-reported spam and never reach 0.3%. Yahoo bulk senders should remain below 0.3%.

Use Google Postmaster Tools for Gmail visibility, Yahoo Sender Hub and its Complaint Feedback Loop for Yahoo complaints, and analyze Outlook rejection and bounce codes.

Email API launch checklist

Domain authentication

• The business owns the sending domain.
• The From address is clear and consistent.
• SPF is published without conflicts.
• DKIM signs every relevant stream.
• DMARC is published and reports are monitored.
• SPF or DKIM aligns with the From domain.
• Forward and reverse DNS are valid.
• Email is transmitted over TLS.

Sending-stream design

• Transactional and marketing email are separated.
• Important use cases have dedicated streams.
• Security messages receive clear priority.
• Marketing campaigns do not use the billing stream.
• Sending rules are defined for each message category.

List management

• The source of every recipient is documented.
• Consent records can be demonstrated.
• Unsubscribes are synchronized.
• Hard bounces are automatically suppressed.
• Spam complaints stop future sending.
• Inactive recipients follow a defined sunset policy.

API integration

• Webhooks receive deliveries, bounces and complaints.
• Hard and soft bounces are classified separately.
• Every message has a unique identifier.
• Retry processes are limited.
• An application failure cannot send duplicate messages repeatedly.
• API credentials are stored securely.
• API-key permissions are restricted.
• Event logs avoid unnecessary personal or sensitive data.

Testing

• Messages are tested with Gmail, Outlook and Yahoo.
• Mobile and desktop rendering is reviewed.
• HTML and plain-text versions are valid.
• Links work and use trusted domains.
• Subject lines are not deceptive.
• The sender identity is clear.
• Unsubscribe works correctly.
• Images and attachments are not unnecessarily large.

Common email deliverability mistakes

Treating SPF as sufficient

SPF does not sign message content and does not independently satisfy every From-domain requirement. Businesses need DKIM, DMARC and alignment.

Publishing DMARC without reviewing reports

A p=none policy does not fully protect the domain. It should be used to discover sending sources before moving to a stronger policy.

Using one identity for every message

A weak marketing campaign can damage invoice and security-email delivery when both use the same reputation.

Purchasing an email list

A large unwanted list creates complaints and bounces faster than it creates sustainable business results.

Hiding unsubscribe

A recipient who cannot leave easily may use the spam button instead.

Increasing volume suddenly

Unexpected volume spikes can cause filtering, throttling or rejection.

Measuring opens only

Teams should review complaints, bounces, deferrals, rejections and completed business outcomes, not only opens.

Assuming delivered means inbox

Server acceptance does not prove that the recipient saw the message in the inbox.

How OurSMS supports business email

OurSMS provides Email API as part of its business communication services, alongside SMS, SMS API, WhatsApp Business API and OTP.

A successful Email API implementation requires more than a successful API request. The business must authenticate its domain, design bounce and unsubscribe handling, separate sending streams and monitor deliverability.

The OurSMS company profile describes API and SMPP integration with applications, CRM systems, ERP platforms and online stores, together with reporting and account-management capabilities.

Email API can support:

• Order confirmations.
• Invoices and receipts.
• Account activation.
• Shipping updates.
• Security alerts.
• Password resets.
• Application notifications.
• Permission-based marketing campaigns.

Email can also work with SMS for urgent fallback alerts, WhatsApp Business API for interactive conversations and OTP for user verification.

Conclusion

Inbox placement in 2026 is not created by design and copy alone.

Businesses need:

• SPF, DKIM and DMARC.
• From-domain alignment.
• Healthy domain and IP reputation.
• Separate transactional and marketing streams.
• Permission-based recipient lists.
• Fast and simple unsubscribe.
• Automated bounce and complaint processing.
• Gradual sending-volume growth.
• Continuous monitoring by mailbox provider.

Start with authentication, clean the list, separate traffic and monitor reputation.

Every unwanted email can affect more than one campaign. It can reduce the ability of your business domain to deliver invoices, security alerts and other important customer communication in the future.